1. Field of the Invention
The present invention relates to the field of a liquid crystal pixel array on a microprocessor and more particularly to a process of preventing corrosion of the metal pixel array.
2. Discussion of Related Art
Silicon-based microprocessors that are used to create and project images for digital high definition televisions (HDTV) have an array of highly reflective metal pixels formed over a monocrystalline silicon substrate. Liquid crystal is encased over the pixels. This is called liquid crystal on silicon (LCOS) display technology for digital televisions because the liquid crystal projects an image formed by input from the metal pixels that are controlled by the microprocessor formed of transistors manufactured in a layer beneath the pixel array on the surface of the silicon substrate.
The highly reflective metal pixels are formed of an aluminum or an aluminum and copper alloy film that has been etched. An aluminum or aluminum/copper alloy film is formed on a silicon wafer. A pattern for the etching is formed with a photoresist material that has been irradiated through a mask to form the patterns and the photoresist is then developed. The photoresist pattern serves as a mask when the metal film is etched. The metal film is etched with a chlorine plasma etchant that severely corrodes the aluminum or aluminum/copper film. The corrosion is caused by the combination of the chlorine in the etchant with air that form hydrochloric acid. The corrosion may form peaks that are above 1 micron high and may also distort and destroy the metal film. After etching an aluminum/copper alloy film into a pixel array, the pixel array is cleaned with a polymer removal chemical that may contain ethylene glycol to remove the polymer-based photoresist. Such polymer removal chemicals containing ethylene glycol causes pitting corrosion of the aluminum/copper alloy film. FIG. 1 illustrates a silicon substrate 100 on which an array of metal pixels 110 has been formed from a thin aluminum copper alloy layer 120 by the method described above. Corrosion peaks 130 and metal pitting corrosion 140 within the grain boundaries of the metal pixels 110 result, as described above.
The corrosion of the aluminum/copper alloy pixel array degrades the yield and performance of the LCOS microprocessors used in digital televisions. This is because the pixels must be highly reflective to work properly. Also, the planarity of the pixels is necessary for device performance and to permit the placing of a glass lid over the pixels to create a sealed space between the pixels and the glass lid into which the liquid crystal is inserted. If there are corrosion peaks having a height of greater than the thickness of the space between the pixels and the glass lid, the glass lid will not fit properly and the liquid crystal will leak out.